Ion detectors including a combination of a conversion dynode and a secondary electron multiplier are often used for detecting ions with high sensitivity in a mass spectrometer. In such an ion detector, a high voltage (± several [kV] to ±10 [kV], for example) having a polarity opposite to that of the ions to be analyzed needs to be applied to a conversion dynode in order to selectively detect positive ions and negative ions. In a liquid chromatograph mass spectrometer, an ion source according to an electrospray ionization (ESI) method, for example, is used for vaporizing and ionizing a liquid sample. In such an ion source, a high voltage (± several [kV], for example) having the same polarity as that of the ions to be analyzed needs to be applied to the tip of a nozzle for spraying the liquid sample.
In these applications, the polarity of the high voltage to be applied needs to be changed in accordance with the polarity of the ions to be analyzed. Therefore, a high-voltage power unit capable of switching the polarity of an output voltage for one line is used. One of the conventionally known high-voltage power units for switching high voltages having different polarities is a power unit using a high-voltage reed relay (see, for example, Patent Literature 1).
In such a high-voltage power unit using a reed relay, in order to avoid a damage to the relay due to possible spiked discharges at the time of switching the polarity of an output voltage, it is necessary to take the following procedures: decrease the output voltage having one polarity; operate the relay to change the contacts once the output voltage becomes adequately low; and, subsequently, increase the output voltage having the other polarity. Consequently, it takes some time to switch the polarity. In the case where, for example, positive ion detection and negative ion detection are alternately performed every short period of time, an ion non-detection period increases in a mass spectrometer. This causes a problem of affecting the accuracy of an analysis.
As a solution to such a problem, a high-voltage power unit capable of switching the polarity of an output voltage at high speed is disclosed in Patent Literature 2. FIG. 5 is a circuit configuration diagram of a principal part of the high-voltage power unit, and 6A and FIG. 6B are waveform charts illustrating change in a voltage in the case of polarity switching in the high-voltage power unit. With reference to FIG. 5, FIG. 6A, and FIG. 6B, a configuration and operation of the high-voltage power unit is schematically described.
In the high-voltage power unit illustrated in FIG. 5, a positive voltage generating unit 2 includes: a booster transformer T1; a drive circuit 3 for driving a primary winding of the booster transformer T1; and a rectifier circuit using a Cockcroft-Walton circuit composed of four capacitors C1 to C4 and four diodes D1 to D4 connected to a secondary winding of the booster transformer T1. A negative voltage generating unit 4 is similar in a basic configuration to the positive voltage generating unit 2 except for the fact that each of diodes D5 to D8 in a Cockcroft-Walton circuit is oriented opposite to that in the positive voltage generating unit 2.
An output terminal P2 of the positive voltage generating unit 2 and an output terminal Q1 of the negative voltage generating unit 4 are connected. Another output terminal Q2 of the negative voltage generating unit 4 is grounded. Between the output terminals P1 and P2 of the positive voltage generating unit 2, a resistor 6 is connected in parallel. Between the output terminals Q1 and Q2 of the negative voltage generating unit 4, another resistor 7 is connected in parallel. A high voltage Vout whose polarity is switched is output from the output terminal P1 of the positive voltage generating unit 2. Between this high-voltage output terminal and the ground, a resistor 8 and a resistor 9 are connected in series. A voltage signal is fed back to a controlling unit 1 from a junction point between the resistors 8 and 9.
The drive circuits 3 and 5 each include a direct current voltage supply, which is connected in series to the primary winding of the booster transformer T1, and a switching element. The voltage applied (or the current supplied) from the direct current voltage supply to the primary winding is connected and disconnected by the switching element. The pulse width of a rectangular wave signal for ON/OFF driving of the switching element is adjusted based on a signal given by the controlling unit 1. Accordingly, the effective electric power supplied to the primary winding of the booster transformer T1 is changed, and consequently output voltages of the positive voltage generating unit 2 and the negative voltage generating unit 4 are changed.
To output a positive high voltage +HV, based on a polarity switching command signal (not illustrated), only the drive circuit 3 in the positive voltage generating unit 2 is operated, and the drive circuit 5 in the negative voltage generating unit 4 is stopped. At this time, since a voltage value corresponding to the voltage +HV appearing at the high-voltage output terminal is fed back to the controlling unit 1, the controlling unit 1 compares this voltage value with a target control voltage and adjusts the signal supplied to the drive circuit 3 so as to reduce an error between the compared voltages. Accordingly, the output voltage +HV is precisely set to any target voltage. Contrary to the above case, to output a negative high voltage, only the drive circuit 5 in the negative voltage generating unit 4 is operated, and the drive circuit 3 in the positive voltage generating unit 2 is stopped.
During a transition period in which output of the positive high voltage +HV is switched to output of a negative high voltage, the controlling unit 1 controls each of the drive circuits 3 and 5 such that the output of the positive voltage generating unit 2 changes from the voltage +HV to zero while simultaneously the output of the negative voltage generating unit 4 changes from zero to subside on a voltage −HV after a overshoot (see waveforms (a) and (b) in FIG. 6A). Thus, by deliberately overshooting the voltage whose absolute value rises from zero in this way, a slow fall of the other voltage that returns to zero is compensated for. This enables the output voltage Vout to promptly reach a target voltage. Accordingly, the output voltage Vout is switched in a short period of time.
Such shortening of the polarity switching time using the deliberate overshoot as described above is significantly effective when a voltage defined as a rated output is output as illustrated in FIG. 6A (in this example, when the rated output voltage is ±10 [kV] and the output voltage Vout is ±10 [kV]), and the output voltage Vout is smoothly switched. However, because the overshoot voltage is optimized for the rated output, in the case where an actual output voltage is lower than the rated output voltage, the overshoot is excessive, so that the time required for the output voltage Vout to become stable is adversely longer.
FIG. 6B illustrates an example in which the rated output is ±10 [kV] and the output voltage Vout is ±5 [kV]. In this example, the positive output voltage or the negative output voltage greatly overshoots at the time of polarity switching. This affects the output voltage Vout, so that the polarity switching time is much longer than that in the case of the rated output.